Yeah it's a touchy subject.
The best is just to say "you're a gorgeous/beautiful woman". That's all that matters.....
In fact, in person, never approach a transgender person and say "you're so brave or you look so female", etc. There's really no good thing to say to a stranger like that because we all just want to be seen as female and saying that we look "so female" tells us you knew about our past maleness by looking at us so in fact, we did not look female enough.